But a NASA-funded research team has added an amazing new twist to the idea; that Earth wasn't just sideswiped by Theia, as many have conjectured, but sustained a direct hit and actually absorbed a large portion of the rogue planet.

"Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them," lead author Edward Young, a professor at the University of California, Los Angeles, said in a statement.

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New research suggests the Earth is made up of two planets that collided and fused together 4.5 billion years ago.

For their study, the UCLA-led team compared the chemical signatures of moon rocks with volcanic rocks from Hawaii and Arizona.

If Earth and Theia had collided in a glancing side blow, as the giant impact hypothesis suggests, the moon would be mostly made up of Theia, and the Earth and moon would have different chemical structures, UCLA said in its statement.